This invention relates to a communication interface and more particularly to a communication interface of a simple structure for a device adapted to be connected to an apparatus provided with a communication port comprising a balanced-type interface circuit.
An apparatus having a standard RS422 communication port may be considered an example of apparatus provided with a balanced-type interface circuit. RS422 is an interface standard established mainly by the United States Electronic Industries Association (EIA) and a data communication system based on the RS422 standard is schematically described in FIG. 3 wherein numberal 10 indicates a communication apparatus provided with an RS422 communication port and numeral 30 indicates a peripheral device to be connected to the communication apparatus 10 through a communication cable 20. The communication apparatus 10 includes a central processing unit CPU 11, which controls the overall operation thereof, balanced-type receivers 12.sub.1 and 12.sub.3 and balanced-type drivers 12.sub.2 and 12.sub.4, the receivers 12.sub.1 and 12.sub.3 and the drivers 12.sub.2 and 12.sub.4 forming a communication interface circuit of the apparatus 10. The peripheral device 30 likewise includes a CPU 31 which controls the overall operation thereof, and also drivers 32.sub.1 and 32.sub.3 and receivers 32.sub.2 and 32.sub.4 form pairs with those in the interface circuit of the communication apparatus 10 and also comprise together a communication interface circuit of the peripheral device 30. Groups of resistors R.sub.1, R.sub.2, and R.sub.3, R.sub.4, R.sub.5 and R.sub.6, R.sub.7, R.sub.8 and R.sub.9, and R.sub.10, R.sub.11 and R.sub.12 are connected respectively to the input end of the receivers 12.sub.1, 12.sub.3, 32.sub.2 and 32.sub.4 to form so-called fail-safe circuits for preventing instability of the input signals to the receivers 12.sub.1, 12.sub.3, 32.sub.2 and 32.sub.4 when the transmission line has been broken unexpectedly by an accident or the connector has become disengaged. Numeral 13 indicates a connector for connecting the communication interface circuit of the apparatus 10 to cable 20. Since at least about ten signal lines are required for RS422 communication, a D-type subconnector with 25 pins is usually used as the connector 13. Numeral 33 indicates another connector for connecting the communication interface circuit of the peripheral device 30 to the cable 20. A D-type subconnector with 25 pins is similarly used for this purpose.
Communication between the devices described above is explained next by way of FIG. 4. In FIG. 4, charts (a)-(d) relate to the communication apparatus 10 and charts (e)-(h) relate to the peripheral device 30. With respect to FIG. 3, the RTS terminal of the CPU 11 for the apparatus 10 is for requesting transmission of a signal and it is normally in an active condition (or in "L" level) as shown in FIG. 4(a). Charts (a), (b), etc. in FIG. 4 are hereinafter referred to as FIG. 4(a), FIG. 4(b), etc., respectively, for convenience. The RTS signal (that is, the signal transmitted from the RTS terminal) is transmitted to the peripheral device 30 from the driver 12.sub.4 through the cable 20. The RTS signal thus transmitted is received by the receiver 32.sub.4 of the peripheral device 30 and is applied to the CTS terminal of the CPU 31 to indicate that communication can now be started. Thus, as shown in FIG. 4(f), the CTS terminal is normally also in an active condition (or in "L" level) and a data signal is transmitted from the TXD terminal, which is a data output terminal, of the CPU 31 when the CTS terminal is in an active condition as shown in FIG. 4(g). This data signal is transmitted from the driver 32.sub.1 through the cable 20 to the communication apparatus 10 and is received as shown in FIG. 4(d) by the receiver 12.sub.1 of the apparatus 10 to be applied to the RXD terminal which is a data-receiving terminal of the CPU 11.
When the peripheral device 30 makes a request for transmission of a signal from the communication apparatus 10, the RTS terminal, which is a transmission-requesting terminal of the CPU 31 on the side of the peripheral device 30 becomes active as shown in FIG. 4(e). This RTS signal is transmitted from the driver 32.sub.3 through the cable 20 to the communication apparatus 10 and received by its receiver 12.sub.3 to be applied as shown in FIG. 4(b) to the CTS terminal of the CPU 11 for indicating that communication can now be started. The CPU 11 outputs data signals from its TXD terminal as shown in FIG. 4(c) while its CTS terminal is in an active condition. The signal thus transmitted is received by the receiver 32.sub.2 of the peripheral device 30 to be applied to the RXD terminal of the CPU 31 as shown in FIG. 4(h).
Data transmission between devices provided with such balanced-type interface circuits suffers less from the effects of noise than similar data transmission between devices with interface circuits of known types and hence is better suited for long-distance communications. In order to connect a communication apparatus provided with a balanced-type interface circuit with a peripheral device, however, such a peripheral device must also have an interface circuit of the RS422 type. Unfortunately, RS422 interface circuits are structurally complicated and they are expensive even if dedicated ICs are used. Where the distance to the peripheral device with which only simple communications are to be made is small and the effects of noise are not important, in particular, it is not economical to convert the interface circuits of peripheral devices into balanced-type ones simply because the principal communication apparatus happens to have a balance-type interface circuit such as RS422.